1.1. Radon in Indoor Air
Radon-222 has been identified as a pervasive pollutant in indoor air. Radon is now known to be responsible for the majority (55%) of the ionizing radiation dose which individuals receive from all sources --both natural and man-made--and it has been amply demonstrated that exposure to elevated levels of this gas in household air is associated with increased incidence of lung cancer (1).
Radon (Rn) is a naturally occurring element that is formed upon the radioactive decay of radium-226. It exists as a gas in the form of three isotopes--Rn-219, Rn-220, and Rn-222. The former two isotopes have half-lives of the order of seconds and thus are of little concern. However, radon-226 decays in a slower process that is characterized by a considerably longer half-life of 3.82 days. Radon decay proceeds with emission of alpha particle radiation through a series of solid, short-lived radioisotopes (e.g., polonium-218 and polonium-214) that are collectively referred to as radon "daughters" or progeny.
These radon daughters, which are unstable isotopes in their own right, are responsible for most of the radiation dose associated with high radon levels in air. Most radon gas that is inhaled is generally exhaled as well since its radioactive half-life is long as compared to the residence time of the gas in the lungs. However, the above-mentioned alpha-emitting polonium isotopes are solids rather than gases, and a fraction of these radon daughters are deposited on the surfaces of the airways deep in the lung when air is inhaled. The radon progeny deposited in this manner subsequently decay by emission of short-range but slow-moving and powerful alpha particles capable of damaging cells which they encounter. This alpha radiation dose is efficiently delivered to the cancer-generating stem cells present in the epithelium that comprises the surface of the air passages in the lungs.
Early EPA estimates indicated that in the United States alone some 5,000 to 20,000 lung cancer deaths a year are attributable to "natural" radon from all sources, and more recent estimates tend to be larger--e.g., between 10,000 and 40,000 lung-cancer deaths each year (2,3). Extensive surveys of radon levels in homes and schools are underway at EPA's urging, based on its finding that some 10% of the nation's homes exceed its 4 picoCuries per liter (pCi/L) action level (4,5). Congress has recently identified a long-term goal of reducing indoor radon concentrations to typical levels in the outside environment (0.1-0.7 pCi/L), and EPA is publicizing Rn mitigation measures (6) and establishing the groundwork for eventual regulations dealing with allowable levels of radon in indoor air and drinking water.
The radon that ultimately enters the home emanates from one of three potential sources: (i) diffusion from soil gas; (ii) release from radon-laden groundwater; and/or (iii) release from radium-containing construction materials (7). The invention disclosed herein addresses the contribution to indoor air pollution from the second source--namely, waterborne radon.